1. Field of the Invention
The present invention relates to a semiconductor memory device and a driving method thereof.
2. Related Art
In recent years, there are FBC memory devices as a semiconductor memory device expected to be an alternative to 1T (Transistor)-1C (Capacitor) DRAMs as a memory. The FBC memory device has an FET (Field Effect Transistor) formed to include a floating body (hereinafter, also called “body”) on an SOI (Silicon On Insulator) substrate. The FBC memory device stores data “1” or data “0” based on the number of majority carriers accumulated in this body. For example, in an FBC including N-FETs, a state of a large number of holes accumulated in the body is set as data “1”, and a state of a small number of holes accumulated in the body is set as data “0”. A memory cell storing the data “0” is called a “0” cell, and a memory cell storing the data “1” is called a “1” cell.
The FBC is better than conventional DRAMs for downsizing. However, electrostatic capacitance of the body accumulating a charge is smaller than electrostatic capacitance of a capacitor of the conventional DRAM. Therefore, a data holding time of the FBC is shorter than that of the DRAM, although a leak current from the body of the FBC is smaller than a leak current from a capacitor of the DRAM. Accordingly, a refresh operation needs to be performed frequently. As a result, a proportion of time (refresh busy rate) for prohibiting reading to the outside/writing from the outside becomes large. Further, the power necessary to hold data becomes larger than that of conventional DRAMs. Particularly, large power consumption in a portable device is a significant problem.
Conventionally, in a refresh method using a charge pumping phenomenon, a gate voltage equal to or smaller than a threshold voltage is applied to the “0” cells. Therefore, a channel is not formed, and electrons are not sufficiently trapped in a surface state between a gate dielectric film and the body. Consequently, the “0” cells are not sufficiently refreshed (see U.S. Pat. No. 6,982,918).